Gas turbines, during such a partial load operation, typically follow a pre-programmed control system with suitable control parameters which for example can also be provided as an operating curve over a greater operating range. Therefore, for example during a so-called power operation, that is to say an operation during which the delivered power output of the gas turbine is provided as a control parameter, a set of operating curves can be predetermined in the control system in which predetermined temperature values are associated with selected power output values of the gas turbine. This temperature value corresponds to a temperature value which is relevant for determining the power output of the gas turbine, wherein this for example can be the turbine inlet temperature or the turbine exit temperature. The relevant operating curves serve as reference values during the power operation, by means of which a desired power output value can be established. In other words, these operating curves serve as a set of control setpoint values to which the gas turbine control system adjusts the power output value.
In addition to such temperature values, such operating curves, however, can for example also include operating parameters with regard to the position of a variable compressor guide baffle (compressor guide vanes), as well as other operating parameters, for example a quantity of water injected into the gas turbine or the distribution of the fuel quantity to individual burners or burner stages in the gas turbine in order to suitably adjust the power output of the gas turbine.
The operating curves are typically established individually in the gas turbines for the most diverse operating behaviors and stored in the control system of the gas turbine. The operating curves are determined in this case in such a way that at each point of the operating curve a stable operation of the gas turbine can be carried out. In particular, the specific distribution of fuel to different burners of the gas turbine is stored in the individual operating curves as an operating parameter and typically does not alter, or alters only marginally, inside an operating curve. If now the operation of a gas turbine is to be altered along a predetermined first operating curve to the effect that this is carried out according to a second operating curve which deviates from the first, a change between these operating curves is required. If the two relevant operating curves lie comparatively close together, a direct changeover between first operating curve and second operating curve can possibly be undertaken without instabilities in the operation of the gas turbine being expected. If, however, the two operating curves lie comparatively far apart, the gas turbine, on account of this change, can be transferred into an unstable operating range which in the worst case even leads to an emergency shutdown of the gas turbine.
In the prior art, such as in EP 2 071 157 A1, the change between such operating curves is indeed described but according to this prior art the problems of unstable operating modes during the change between individual operating curves cannot be adequately taken into account. Therefore, for example EP 2 071 157 A1 teaches an optional change between individual operating curves but without specifying that an unstable operation can be the consequence on account of the new operating parameters possibly applying to the new operating curve.
Particularly on account of the different distribution of fuel to the individual burners or burner stages of the gas turbine between two operating curves, during a change between two operating curves comparatively abrupt changes in the combustion chamber can take place, which allow the gas turbine operation to at least temporarily become unstable. During operation along an operating curve, an operating point is indeed typically kept largely stable as a result of suitable controlling of the gas turbine, but this controlling is for the most part only designed for the compensation of smaller deviations. Small deviations along an operating curve mostly behave in a problem free manner in this respect. However, deviations which during the operation occur over a multiplicity of operating curves can sometimes adjust the gas turbine control system without freedom of problems.
DE102006008483A1 teaches another approach to this, according to which valve position command values of fuel flow quantity control valves are to be determined based on the fuel gas ratio (such as a pilot ratio, a cylinder ratio and a main ratio) of a gas turbine. In this case, the opening degree of inlet guide vanes can for example also be calculated in a targeted manner by means of linear interpolation so that a turbine inlet temperature of 1500° C. results. For the interpolation, provision is made in this case for table values which at points relate certain opening degrees to the power output of the gas turbine and to the inlet air temperature. However, disadvantageous to this approach is that no operating curve dependent operation of the gas turbine is enabled.